This invention relates to an electronic semiconductive component comprising at least one layer (2,3) of a p-type or n-type material, wherein the layer of a said p- or n-type material is constituted by a metal hydride having a chosen dopant. The invention also relates to methods for producing the component.
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1. Electronic semiconductive component comprising at least one layer of a p-type and n-type material, said component comprising a p-type and an n-type material constituting a p-n junction, wherein the p- and n-type materials constituting the p-n junction are semiconductive metal hydrides having a chosen dopant.
2. Component according to
3. Component according to
4. Component according to
5. Component according to
6. Component according to
7. A solar cell comprising a component according to
8. Method for producing a component according to
9. Method according to
10. Method according to
11. Method for producing a component according to
12. Method according to
13. Method according to
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This application is the U.S. national phase of International Application No. PCT/NO2009/000045 filed 9 Feb. 2009 which designated the U.S. and claims priority to U.S. Provisional Application No. 61/064,004 filed 11 Feb. 2008 and NO Patent Application No. 20080769 filed 12 Feb. 2008, the entire contents of each of which are hereby incorporated by reference.
This invention relates to a semiconducting component and a method for producing such a component, more specifically it relates to the use and production of metal hydrides as semiconducting devices such as diodes or transistors.
Semiconductors have been commonly used in a wide range of applications for the last 50 years. Most of these are based on well known semiconducting materials such as Si or GaAs doped with different materials in order to adjust the band widths and thus the semiconducting properties of the materials, e.g. for providing p- or n-type conductivity and combinations of such for producing diodes, transistors etc.
A problem with the industry is that even the most common material Si, is a limited resource and the price for Si wafers for semiconductor production has been fairly constant for years. Thus large structures such as solar cells are expensive, and as the production of silicon is energy consuming the environmental gain of solar cells is limited.
Thus there is a need for alternative materials which can be used in semiconductive devices, especially large structures such as solar cells. Recently it has been discovered that some metal hydrides have semiconductive properties, as discussed in the article by S. Karazhanov, P. Ravindran, P. Vajeeston, A. Ulyashin, “Hydride Electronics”, Phys. Stat. Sol. (a) 204 (2007) p. 3538-3544. As discussed in this article the metal hydrides to date has been aimed at fast absorption and desorption of hydrogen, which is advantageous when used as storage means for hydrogen but a disadvantage for electronic applications where stability is an important issue.
The use of hydrogenated materials in solar cells and other electronic devices is well know as such, and some examples are found in the following publications, but mostly used to avoid disadvantages of silicon based solutions.
EP0233613 by C. R. Dickson 1987 Aug. 26 discusses Si (semiconductor) based hydrides and it is related to the discovery of “Giant Micro-electronics”, which is about applications of hydrogenated amorphous silicon (a-Si:H) in semiconductor electronics. Nowadays some other hydrogenated semiconductors such as, e.g., a-Si:H, Si2N3H, etc., are widely used in electronic devices, but they have the same disadvantages as other silicon based solutions.
Patent application WO 9927561A by M. Todd et. Al. 1999 Jun. 3 provides solution of the problem as to how to stabilize a hydride to make it suitable for electronic device applications. CN 1483724 by T. Utang et. al. 2004 Mar. 24 discusses hydrogen containing polymer Tris(8-hydroxyquinolinato) aluminum obtained by using the hydride LiAlH4.
FR 2303385 A by J. Meuleman 1976 Oct. 1 discusses a detector consisting of a p-i-n diode from the Si/Hydride device structure where a LiAlH4 is deposited on a Si substrate. One disadvantage of Si(Li) detectors is that they have to be stored in liquid nitrogen or the lithium will diffuse out of the depletion layer.
This object is thus to provide a cost effective solution for making semiconductive devices and is obtained by a method and component as mentioned above and characterized as specified in the accompanying claims.
The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples.
Analysis of literature and our studies of electronic structure and optical properties of hydrides with/without defects and impurities shows that some hydrides are insulators with large fundamental band gap (see
The calculated band gaps presented in
Band structure studies for some of the hydrides (
Total density of states (DOS) N(E) has been calculated (
which have been used to calculate intrinsic carrier concentration ni
Here E is the energy, Ec and Ev are the energies corresponding to bottommost CB and topmost VB. k is the Boltzmann constant and T is the temperature. V0 is the volume of the unit cell and Eg=Ec−Ev is the fundamental band gap. Knowledge of ni allows to estimate diffusion potential of the p-n junction with concentration of shallow donors Nd and shallow acceptors Na
which is one of the important parameters of semiconductor p-n junction based devices. Using the values of N(E) and Eq. (1)-(3), effective DOS for CB (Nc) and VB (Nv) as well as intrinsic carrier concentration (ni) have been calculated for Si, Mg2NiH4-c, and Mg2NiH4-m for T=300 K (Table II). These values are close to those of Si. The small difference can be related to peculiarities of the band structure and fundamental band gap.
The orbital and site projected density of states (PDOS) and fat band analysis has been performed.
One of the parameters characterizing transport properties of solids is carrier effective masses. Effective masses have been calculated for Si and Mg2NiH4-c, and Mg2NiH4-m as illustrated in the table in
Application of a material as electric and optoelectronic devices critically depends on dopability, which can be limited by the following three main factors:
In this specification the issue (ii) of whether an impurity form shallow energy level in the band gap of hydrides.
As the shallow-level defects and impurities can be the source for electrical conductivity of semiconductors, we have calculated electronic structure and optical properties of several hydrides.
Doping can affect not only conductivity but also transparency. In order to establish this viewpoint we have studied absorption and reflectivity spectra of Mg2NiH4-c with AlMg and LiMg (
In TCOs it is well known that one can easily bring in n-type conductivity, but the introduction of p-type conductivity is problematic. It follows from our results that high p-type doping level in hydrides shall not loose transparency, but can provide good conductivity, whereas high n-type doping leads to loss in transparency. This is one of the distinguishing features between TCOs and hydrides. The calculated formation energies for the Li(Al) doped Mg2NiH4-c is −0.20(+1.57) eV, which shows that the configuration with Li is stable, whereas that with Al is not stable.
Knowledge of intrinsic carrier concentration and concentration of shallow acceptors and donors allows one to calculate by Eq. (4) diffusion potential VD. Below we perform such estimation for Mg2NiH4-c. Assuming Nd˜1020 cm−3 and Na˜1015 cm−3 one can find that VD˜1.06 V.
As can be seen from
Although wide band gap hydrides can be conductive, as it was shown above, the doping limit for each concrete hydride material is not known yet and has to be studied systematically. In case if an effective high level doping and electrical conductivity can be realised for some of them along with transparency to the visible part of the solar spectra, they can be regarded as alternative to TCO materials. Nevertheless, at this stage of investigations it is possible to claim already that hydrides are more flexible materials than SiNx or TCOs, which are widely used as antireflection coatings in solar cells. As it is well known, optical properties of SiNx and TCOs and passivation of interface states shall be controlled by modulation of the nitrogen or oxygen stoichiometry, which is rather problematic. Upon using hydrides these features can relatively easily be controlled by hydrogen content only. Consequently, formation of nano-dots and other nano-structures in hydrides can be realized more easily than those in SiNx and TCOs. Furthermore, structural and morphological modifications of hydrides can be done at much lower temperatures and pressures than SiNx and TCOs. Hence, hydrides can be more suitable for low temperature synthesis technologies.
The usage of TCOs in some devices results in the formation of an intermediate oxide layer, large band-offset, and defect states, which can limit the device performance. If hydrides would be used instead of the TCOs, then no oxide layer would be formed and there is no need in using buffer layer. Large band-offset can be prevented by reducing the band gap of the hydrides, which is much easier than that in oxides. Furthermore, hydrogen diffusion from hydrides into semiconductors can be useful to passivate defect states at the interface and increase their conductivity those are important for improving the device performance. Hydrides can also be regarded as a natural alternative to SiNx and TCOs in case of any low temperature technology.
Smaller band gap hydrides with well dispersive bottommost conduction band and topmost valence band can be useful to replace semiconductor devices such as, say, solar cells. From this point of view, hydrides can be regarded as novel class of solar cells. The crystalline nature and light weight of hydrides, as well as small concentration of defects between active solar cell and TCO present interest for third generation photovoltaics. World wide lot of attention is currently focused on developing environmentally friendly technologies, alternative energy sources, improving the performance of solar cells etc. and our finding is directly relevant to these activities. It has to be noted that hydrides in most cases are much lighter than conventional semiconductors, and hydrogen is the most abundant element in the universe. Consequently, hydrides are of great interest for terrestrial and space applications of solar cells, and also hydrides have large potential for applications in bio-compatible semiconducting devices.
The component according to the invention is illustrated in
After the deposition of the capping layer 1 the component is hydrogenated by subjecting the component to a gas flow 5 comprising hydrogen, as will be discussed more in detail below. Resulting from this is a component comprising a capping layer 1, a doped metal hydride layer 2,3 and a substrate. In the example illustrated in
A principle scheme for fabrication of individual n- or p-type metal hydride layers or a p-n (n-p) junction based metal hydride structures can be described as follows:
An alternative production method may be performed by growing the layers as hydrides, which will reduce the stress in the material which occurs when it is loaded with hydrogen after the deposition. A more detailed description is given in the thesis of Ingrid A. M. E. Giebels “Shining light on magnesium based switchable mirrors” ISBN 90-9018547-X, section 2.2.1 where a GdMg—H5 layer is produced by co-sputtering of the metals in a hydrogen/argon atmosphere at room temperature. In this case also the capping layer may be omitted during production. The abovementioned thesis in sections 2.2.2 and 2.2.3 also discloses other production methods incorporating In-Situ and Ex-Situ hydrogen loading. A combination of the production methods may also be contemplated, where the layers are grown as hydrate metals, and additional hydrogen are loaded afterward. This is discussed in section 5.14 in the thesis of Ruud Westerwaal, “Growth, microstructure and hydrogenation of Pd-catalyzed complex metal hydride thin films”, ISBN-978 90 8659 071 1.
Due to a possibility to fabricate metal hydrides with any desirable band gap, which depends on the composition (including the hydrogen content) the individual n- or p-type metal hydrides layers can be utilized for instance as buffer layers for heterojunction devices, such as solar cells. In this case the device structures look like follows:
The n-type/p-type metal hydride stack can also be utilized as a diode, which is a basic structure of several microelectronic devices, including solar cells. In the later case the metal hydride solar cell structures look as follows: Antireflection coating 1/n-type(p-type) metal hydride (emitter) 2/p-type(n-type) metal hydride solar cell base 3/Pd(Ni) 4/back side metallization 7;
A number of different metal hydrides may be used as n- and p-type semiconductors. Some examples are mentioned below:
Mg2NiH4-c
AlH3 (see the abovementioned article by S. Karazhanov et al, published in Phys. Stat. Sol. “hydride electronics”)
Other combinations of metals and dopants may be contemplated, e.g. combinations of the AlHx and NgNiHx materials mentioned above, thus providing any combinations of n-p, p-n, p-p or n-n junctions, as well as three layered components such as n-p-n and p-n-p constituting transistors. The materials are chosen depending on their stability, especially related to temperature and the band gap as well as the compatibility with other materials in the component and connected instruments and conductors. It is also possible to use materials having amorphous properties with the selected concentrations of hydrogen.
Electrical couplings to conductor leads 6,7 for utilizing the component in an electronic system may also be of different types depending on the compatibility. In general there it is an advantage related to the use of metal hydrides as semiconductors that they provide good electrical contact with other materials and conductor leads. The electrical coupling points may be chosen e.g. to the top metal hydride surface 2 and to the substrate 4.
The substrate 4 is made by any material being compatible with the materials of the deposited layers, such as metal, ceramic, glass, polymers, etc, the choice also depending on the need for e.g. an electrically conductive substrate or for providing electrical conductors on or through the substrate. The substrate 4 may also be a metal or metal to be hydrogenated through the production process, thus being part of the active component.
Thus the component according to the inventions involves a layered structure comprising at least one p- or n-type metal hydride. In the preferred embodiment the component incorporates at least two layers, e.g. providing a p-n junction or three layers providing a n-p-n or p-n-p junction, but other solutions may be contemplated, depending on the use, for example an n-n′ configuration.
Usually the component will include a protective top layer, e.g. a TCO and a lower substrate. Between the layers amorphous layers of metal hydrides or silicon may be used, so as to act as stress relieves or buffer layers, but this may not be necessary if two layers are compatible. A possible component according to the invention for use in solar cells could thus be comprised from the bottom and up by a conductive metal substrate 4, a p-type metal hydride layer 3, an n-type metal hydride layer 2, and a TCO 1 on top, with conductors 5,6 connected to the TCO layer 1 and metal substrate 4. Depending on the available materials and their band gaps other structures may also be possible, as mentioned above constituting p-n-p structures etc.
To summarize the invention thus relates to an electronic semiconductive component comprising at least one layer of a p-type or n-type material, wherein said p- or n-type material is constituted by a metal hydride having a chosen dopant. According to the preferred embodiment the component comprises both a p-type and an n-type material constituting a p-n junction, wherein at least one of said p- and n-type materials is constituted by a metal hydride having a chosen dopant. Thus effectively constituting a diode, which can be used as a solar cell.
A number of material may be used for making the n-type material in the component, such as having at least one following materials Mg2NiH4-c with Al atom doping in a concentration of 1015 to 1022 atoms/cm3, MgH2 with Al atom doping in a concentration of 1015 to 1022 atoms/cm3 or MgH2 with Cs atom doping in a concentration of 1015 to 1022 atoms/cm3. Another possible n-type material is made from AlH3 with atom doping from group IV/II atoms, eg. C, Si, Be, Mg etc in a concentration of 1015 to 1022 atoms/cm3.
Similarly p-type materials may for example be provided by using a material made from Mg2NiH4-c with Li atom doping in a concentration of 1015 to 1022 atoms/cm3, MgH2 with Li atom doping in a concentration of 1015 to 1022 atoms/cm3, or at least one following materials AlH3 with Ca atom doping in a concentration of 1015 to 1022 atoms/cm3.
In addition to the p-n junction a third layer may be added, for example making a p-n-p or n-p-n device.
The component may be produced in different ways for example by ex-situ or in-situ deposition or sputtering techniques. The ex-situ deposition method comprising the steps of depositing at least one layer constituted by a chosen metal having a chosen dopant on a substrate, said dopant providing a n- or p-type conductivity after hydrogenation, providing a capping layer on said at least one layer, and exposing the layers and capping to a gas stream containing a chosen fraction of hydrogen. This method may also include depositing of two metal layers, one of which providing an n-type semiconductor after hydrogenation and the second providing a p-type semiconductor after hydrogenation, thus providing a p-n junction, and depositing a third metal layer providing a second p-n or n-p junction after hydrogenation.
The in-situ production method may comprise the steps of depositing at least one first layer constituted by a chosen metal having a chosen dopant on a substrate, the deposition being performed by a sputtering process in an atmosphere constituted by hydrogen and a nobel gas, e.g. argon, thus depositing a hydrogenated doped metal layer, the dopant providing a n- or p-type conductivity.
As with the ex-situ method this may also include a second step of depositing a second layer with a second dopant being different from said first dopant, as well as a third step adding a third layer. A combination of the two methods may also be used by including a step of depositing a capping layer and exposing said component to a gas stream containing a chosen fraction of hydrogen for additional hydrogen loading of the material.
Karazhanov, Smagul, Ulyashin, Alexander G.
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